Jökull - 01.01.2010, Blaðsíða 121

Holocene surface ruptures in the South Iceland Seismic Zone
animals had perished, and holes that were useful for
trash disposal.
The surface in the SISZ is mainly of two different
types. A good part of the zone is covered by a single
lava flow, the Þjórsá lava with an estimated volume of
15 km3 (Hjartarson, 1988), that issued from the East-
ern Volcanic Zone ∼8000 years ago. Other parts of the
zone have basalts of the Plio-Pleistocene Hreppar for-
mation as bedrock, covered by thin ground moraines
left by the last glaciation. The Þjórsá lava generally
preserves the fracture structures rather well and they
are easily traced in spite of considerable vegetation
and soil that covers parts of the lava. An open fracture
tends to maintain a sinkhole in the soil cover above for
a long time after its formation. Tracing fractures in the
Hreppar formation and the moraine material is much
more difficult. The surface erodes faster and therefore
fracture manifestations tend to get erased. There is,
however, no noticeable difference in the appearance
of the structures if they are exposed. This was clearly
demonstrated in the surface effects of the earthquakes
of 2000 (Clifton and Einarsson, 2005).
STRUCTURES ASSOCIATED WITH THE
SURFACE FAULTS
The seismogenic faults of South Iceland are clearly
exposed at the surface in numerous places and can be
identified by characteristic structures. The best expo-
sures are in areas where the Þjórsá lava forms the sur-
face bedrock layer, also where it is covered by a thin
soil layer. The characteristic structures are also found
in areas where the lava is lacking and glacial deposits
form the surface layer but they are not as well pre-
served there.
En echelon fracture arrays constitute one of the
most characteristic features of surface ruptures of
strike-slip faults. The surface trace is marked by
a deformation zone of obliquely trending fractures.
Left-lateral faults are marked by right-stepping ex-
tensional fractures and right-lateral faults have left-
stepping fractures associated with them. In South Ice-
land most of the identified fault ruptures are associ-
ated with right-lateral faulting. Left-lateral faults oc-
cur but almost always in a subordinate role. The en
echelon structure shows up on many different length
scales. The largest scale is measured in kilometers.
Fault segments of one to a few km length are offset
with respect to each other. This scale can be clearly
seen in the faults in Figure 1. The offset between the
segments is of the order of a few hundred meters to
a kilometer. This offset determines the width of the
fractured zone associated with the underlying fault.
Each segment then displays en echelon structure. In-
dividual fractures of lengths up to tens of meters are
offset with respect to each other, forming en echelon
fracture arrays. The pattern is sometimes repeated
on a meter scale within these arrays. During the re-
cent earthquakes of 2000 and 2008 this pattern was
observed on a finer scale yet (Clifton and Einarsson,
2005), to centimeters and millimeters in soil surfaces
(Figure 2).
Push-up structures are frequently found within
en-echelon fracture arrays, usually bridging the gap
between the tips of adjacent extensional fractures.
They have the form of small hillocks, a few tens of
centimeters to a few meters in height. The horizontal
dimensions and shape are quite variable. Some push-
ups are almost round in shape, others are quite elon-
gated. The tops of the hillocks are usually heavily
fractured, sometimes to the extent of looking crushed.
When fracture systems can be discerned there appear
to be two separate kinds of fractures, longitudinal and
transverse. The longitudinal fractures are sub-parallel
to the adjacent extensional fractures and thus form
a part of the en echelon pattern of the over-all frac-
ture array. There may be several of these crossing the
push-up (Figure 3) resembling a sliced loaf of bread,
often rotated slightly with respect to the adjacent pat-
tern. In a left-stepping, N-S trending array the longitu-
dinal fractures would thus have slightly more easterly
strike than the adjacent extensional fractures.
The transverse fractures usually follow the crest of
the push-up and are extensional in nature. In lava sur-
faces that are not affected by other processes it can be
seen how the transverse fractures occupy the crest of
folded antiforms. The push-up is then seen as a minia-
ture fold with axis perpendicular to the maximum hor-
izontal principal stress. This structure was demon-
strated well in the surface ruptures associated with the
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